The present invention relates to solar battery charging systems and in particular to voltage regulation of photovoltaic recharging of secondary batteries under varient ambient light conditions.
The conventional photovoltaic recharging system is designed to operate within a given insolance range providing a predetermined range of power output. An appropriate array of series and parallel connected photovoltaic cells with known current-voltage characteristics may be constructed to provide the required recharging power for a given range of insolance. Where the maximum value of insolance is ascertainable and the average variance of insolation during the required recharging period is relatively small, the recharging power regulation may be accomplished merely by a judicious selection of the size and number of parallel and the number of serial connected photovoltaic cells. A regulation circuit under such circumstances need only comprise a blocking diode to prevent discharging the secondary battery through the photovoltaic cells. Recharging power would be inherently limited by the capability of the photovoltaic array. However, where the anticipated insolance varies significantly, that is greater than an order of magnitude, and the secondary battery is sensitive to over-voltage charging, cell array design alone will not serve as an efficient power regulating means.
Conventionally, a recharger circuit will be designed to produce the desired recharging power under insolance conditions anticipated during a majority of the rechargers' operation. This type of recharger would typically be designed to optimize the size and number of cells in the array (as opposed to optimizing the use of anticipated ambient light). Such systems are suitable for minor variations in insolance, but if ambient light exposure were to vary significantly, this design would either sacrifice high insolance recharging or would ineffectively contribute to battery recharging at low level illumination. That is to say that the photovoltaic recharger array is typically designed in a manner which would either:
a. Provide sufficient recharging at higher illumination levels, but contribute dimunitive recharging power at room light or similarly low level light intensity; or
b. Provide sufficient recharging power at lower illumination levels and none at higher light intensities.
The present invention relates to a simple regulator circuit which provides recharging at illumination levels varying over two orders of magnitude while providing overpotential charging regulation. The invention is of particular utility in recharger devices which rely on room light irradiance recharging yet often are exposed to higher intensities such as direct sunlight, and all irradiance levels in between. Many devices such as calculators, wrist watches, childrens' toys and the like are exposed to such varied light conditions. The present invention provides recharging ability throughout these illumination ranges yet regulates against battery overcharging which may result in the destruction of the battery.